A typical representation of a switching power supply is depicted in FIG. 1. In the switching power supply of FIG. 1, a rectifier 11 is used to receive an input AC voltage Vin and convert the input AC voltage Vin into a rectified DC voltage. A switch device 12 is coupled between an output end of the rectifier 11 and the primary winding of a transformer T1. The transformer T1 is provided to receive the rectified DC voltage from its primary winding and acts as an energy storage device to store magnetization energy therein while the switch device 12 is turned on. While the switch device 12 is turned off, the magnetization energy reserved in the primary winding of the transformer T1 is transferred across a secondary winding. An output circuit 13 including a rectifier diode D and a smoothing capacitor C 131 is placed between the secondary winding of the transformer T1 and an output terminal of the switching power supply for providing an output DC voltage Vout scaled according to the turn ratio of the transformer T1 to a load 15. It is to be noted that the output power of the switching power supply contributed by the transferred magnetization energy through the transformer T1 is proportional to the operating frequency of the switch device 12. A feedback loop 14 is coupled to the output terminal of the switching power supply for providing a feedback signal indicating the variation of the output voltage Vout and measuring the value of the current flowing through the load 15. The switching power supply of FIG. 1 further incorporates a switching control circuit 16 that is normally implemented by a pulse-width modulation (PWM) controller. The switching control circuit 16 normally includes an error sampling circuit, an oscillator, and a PWM signal generation circuit, all of which are not shown in the drawings for simplicity. The error sampling circuit generally detects an error between the feedback signal from the feedback loop 14 and a predetermined reference signal for the output to the PWM circuit. The oscillator circuit is used to generate an oscillating frequency signal. The PWM signal generation circuit is used to provide a PWM signal based on the oscillating frequency signal from the oscillator circuit and the feedback control signal from the error sampling circuit for regulating the output voltage Vout of the switching power supply. Thus, the output voltage Vout can be stabilized at a predetermined value.
Generally, the maximum output power of a switching power supply is restrained by the limited pulse width of the PWM signal and thus its load driving capability is generally insufficient. When a conventional switching power supply is used in cooperation with a light load device, it can operate at its maximum duty cycle. However, if the load device is a computer peripheral, for example, a printer, there is a general requirement that the switching power supply operates under a peak load condition, which means the switching power supply required to provide two times or more than two times of the continuous power output. Under this condition, a conventional switching power supply is incompetent to drive the load because the load power has exceeded its maximum load driving capability.
There is a tendency to develop a switching power supply which incorporates an oscillator frequency tuner that is competent to enhance the load driving capability of the switching power supply under peak load condition.